CHAPTER 17 FROM GENE TO PROTEIN

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° Wild-type Neurospora can grow on a minimal medium of agar, inorganic salts, glucose,and the vitamin biotin.• Beadle and Tatum identified mutants that could not survive on minimal medium, becausethey were unable to synthesize certain essential molecules from the minimal ingredients.° However, most of these nutritional mutants can survive on a complete growth mediumthat includes all 20 amino acids and a few other nutrients.• One type of mutant required only the addition of arginine to the minimal growth medium.° Beadle and Tatum concluded that this mutant was defective somewhere in thebiochemical pathway that normally synthesizes arginine.° They identified three classes of arginine-deficient mutants, each apparently lacking a keyenzyme at a different step in the synthesis of arginine.° They demonstrated this by growing these mutant strains in media that provided differentintermediate molecules.° Their results provided strong evidence for the one gene–one enzyme hypothesis.• Later research refined the one gene–one enzyme hypothesis.• First, not all proteins are enzymes.° Keratin, the structural protein of hair, and insulin, a hormone, both are proteins and geneproducts.• This tweaked the hypothesis to one gene–one protein.• Later research demonstrated that many proteins are composed of several polypeptides, eachof which has its own gene.• Therefore, Beadle and Tatum’s idea has been restated as the one gene–one polypeptidehypothesis.• Some genes code for RNA molecules that play important roles in cells although they arenever translated into protein.2. Transcription and translation are the two main processes linking gene to protein.• Genes provide the instructions for making specific proteins.• The bridge between DNA and protein synthesis is the nucleic acid RNA.• RNA is chemically similar to DNA, except that it contains ribose as its sugar and substitutesthe nitrogenous base uracil for thymine.° An RNA molecule almost always consists of a single strand.• In DNA or RNA, the four nucleotide monomers act like the letters of the alphabet tocommunicate information.• The specific sequence of hundreds or thousands of nucleotides in each gene carries theinformation for the primary structure of proteins, the linear order of the 20 possible aminoacids.• To get from DNA, written in one chemical language, to protein, written in another, requirestwo major stages: transcription and translation.• During transcription, a DNA strand provides a template for the synthesis of acomplementary RNA strand.IG Lecture Outlines <strong>17</strong>-2
° Just as a DNA strand provides a template for the synthesis of each new complementarystrand during DNA replication, it provides a template for assembling a sequence of RNAnucleotides.• Transcription of many genes produces a messenger RNA (mRNA) molecule.• During translation, there is a change of language.° The site of translation is the ribosome, complex particles that facilitate the orderlyassembly of amino acids into polypeptide chains.• Why can’t proteins be translated directly from DNA?° The use of an RNA intermediate provides protection for DNA and its genetic information.° Using an RNA intermediate allows more copies of a protein to be made simultaneously,since many RNA transcripts can be made from one gene.• Also, each gene transcript can be translated repeatedly.• The basic mechanics of transcription and translation are similar in eukaryotes andprokaryotes.• Because bacteria lack nuclei, their DNA is not segregated from ribosomes and other proteinsynthesizingequipment.° This allows the coupling of transcription and translation.° Ribosomes attach to the leading end of an mRNA molecule while transcription is still inprogress.• In a eukaryotic cell, transcription occurs in the nucleus, and translation occurs at ribosomes inthe cytoplasm.° The transcription of a protein-coding eukaryotic gene results in pre-mRNA.° The initial RNA transcript of any gene is called a primary transcript.° RNA processing yields the finished mRNA.• To summarize, genes program protein synthesis via genetic messages in the form ofmessenger RNA.• The molecular chain of command in a cell is DNA RNA protein.3. In the genetic code, nucleotide triplets specify amino acids.• If the genetic code consisted of a single nucleotide or even pairs of nucleotides per aminoacid, there would not be enough combinations (4 and 16, respectively) to code for all 20amino acids.• Triplets of nucleotide bases are the smallest units of uniform length that can code for all theamino acids.• With a triplet code, three consecutive bases specify an amino acid, creating 4 3 (64) possiblecode words.• The genetic instructions for a polypeptide chain are written in DNA as a series ofnonoverlapping three-nucleotide words.• During transcription, one DNA strand, the template strand, provides a template for orderingthe sequence of nucleotides in an RNA transcript.° A given DNA strand can be the template strand for some genes along a DNA molecule,while for other genes in other regions, the complementary strand may function as thetemplate.IG Lecture Outlines <strong>17</strong>-3
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CHAPTER 17 FROM GENE TO PROTEIN

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